1. Field of the Invention
A toroidal-type continuously variable transmission according to the invention is used as transmission unit constituting an automatic transmission for a car, or, as a transmission apparatus for adjusting the operating speed of various industrial machines such as a pump.
2. Description of the Related Art
Conventionally, to use such a toroidal-type continuously variation transmission as shown in FIGS. 4 to 6 as an automatic transmission apparatus for a car has been studied and such use has been enforced in part of the car industry. A toroidal-type continuously variable transmission of this type is referred to as a toroidal-type continuously variable transmission of a double cavity type, in which input side disks 2, 2 serving as first disks are supported through their respective ball splines 3, 3 on the peripheries of the two end portions of an input shaft 1. Therefore, these two input side disks 2, 2 are supported in such a manner that they are concentric with each other and can be rotated in synchronization with each other. Also, an output gear 4 is supported on the periphery of the middle portion of the input shaft 1 in such a manner that it can be rotated with respect to the input shaft 1. And, output side disks 5, 5 serving as second disks are respectively spline engaged with the two end portions of a cylindrical portion disposed in the central portion of the output gear 4. Therefore, the two output side disks 5, 5 can be rotated together with the output gear 4 in synchronization with each other.
Also, respectively between the mutually associated input side disks 2, 2 and output side disks 5, 5, there are held a plurality of (normally, 2 to 3) power rollers 6, 6. The power rollers 6, 6 are respectively supported on the inner surfaces of trunnions serving as support members through support shafts 8, 8 and a plurality of rolling bearings in such a manner that they can be rotated. The trunnions 7, 7 can be respectively swung and shifted about their associated pivot shafts 9, 9 which are disposed on the two end portions of the trunnions 7, 7 in their respective longitudinal directions (in FIGS. 4, 6, in the upward and downward directions; and, in FIG. 5, in the front and back directions) in such a manner that the pivot shafts 9, 9 are concentric with each other. An operation to incline the respective trunnions 7, 7 is carried out by shifting the trunnions 7, 7 in the axial directions of the pivot shafts 9, 9 using actuators 10, 10 each of an oil pressure type; and, the inclination angles of all of the trunnions are synchronized with each other in an oil pressure manner and in a mechanical manner.
That is, in case where the inclination angles of the trunnions 7, 7 are changed in order to change a transmission ratio between the input shaft 1 and output gear 4, the trunnions 7, 7 are respectively shifted in the mutually opposite directions, using their associated actuators 10, 10; for example, the power roller 6 situated on the right side in FIG. 6 is shifted to the lower side in FIG. 6 and the power roller 6 on the left side in FIG. 6 is shifted to the upper side in FIG. 6. This changes the directions of forces going in the tangential direction which respectively act on the contact portions between the peripheral surfaces of the power rollers 6, 6 and the inner surfaces of the input side disks 2, 2 and output side disks 5, 5 (that is, side-slip occurs in the contact portions). And, with such change in the directions of the tangential-direction forces, the trunnions 7, 7 are swung (inclined) in the mutually opposite directions about the pivot shafts 9, 9 which are pivotally supported on their associated support plates 11, 11. As a result of this, the contact positions between the peripheral surfaces of the power rollers 6, 6 and the inner surfaces of the input side and output side disks 2, 5 are caused to change, thereby changing the rotation transmission ratio between the input shaft 1 and output gear 4.
Supply of pressure oil to the above respective actuators 10, 10 is carried out using a single transmission ratio control valve 12 regardless of the number of these actuators 10, 10; and, the motion of any one of the trunnions 7, 7 is fed back to the transmission ratio control valve 12. The transmission ratio control valve 12 includes a sleeve 14 to be shifted in the axial direction (in FIG. 6, in the right and left direction; and, in FIG. 4, in the front and back direction) by a stepping motor 13, and a spool 15 fitted into the inside diameter side of the sleeve 14 in such a manner that it can be shifted in the axial direction. Also, of rods 17, 17 which connect together the trunnions 7, 7 and the pistons 16, 16 of the actuators 10, 10, to the end portion of the rod 17 which belongs to any one of the trunnions 7, there is fixed a precess cam 18, thereby constituting a feedback mechanism which transmits the above-mentioned motion of the rod 17, that is, the composite value of the axial-direction shift amount and the rotation-direction amount to the spool 15 through the present precess cam 18 and a link arm 19. Also, a synchronizing cable 20 is set between the trunnions 7, 7, thereby being able to mechanically synchronize the inclination angles of the trunnions 7, 7 with each other even in case where an oil pressure system is out of order.
To switch over the transmission state, the sleeve 14 is shifted to a given position corresponding to a transmission ratio to be obtained by the stepping motor 13 to thereby open the given-direction flow passage of the transmission ratio control valve 12. As a result of this, the pressure oil is fed into the actuators 10, 10 in the given direction, so that these actuators 10, 10 shift the trunnions 7, 7 in the given direction. That is, as the pressure oil is fed, the trunnions 7, 7 not only are shifted in the axial direction of the pivot shafts 9, 9 but also are swung about these pivot shafts 9, 9. And, the motion (axial-direction shifting and swinging motion) of the above-mentioned one of the trunnions 7 is transmitted to the spool 15 through the precess cam 18 fixed to the end portion of the rod 17 and link arm 19, thereby shifting the spool 15 in the axial direction. As a result of this, in a state where the present trunnion 7 is shifted by a given amount, the flow passage of the transmission ratio control valve 12 is closed to thereby stop the supply of the pressure oil to the actuators 10, 10.
In this case, the motion of the transmission ratio control valve 12 based on the shifting motion of the trunnion 7 and the shifting motion of the cam surface 21 of the precess cam 18 is as follows. That is, firstly, in case where the trunnion 7 is shifted in the axial direction as the flow passage of the transmission ratio control valve 12 is opened, as described above, due to the side-slip which occurs in the contact portions between the peripheral surfaces of the power rollers 6 and the inner surfaces of the input side and output side disks 2, 5, the above trunnion 7 starts its shifting and swinging motion about its associated pivot shafts 9, 9. Also, with the axial-direction shifting motion of the trunnion 7, the shifting motion of the cam surface 21 is transmitted through the link arm 19 to the spool 15, so that the spool 15 is shifted in the axial direction to thereby change the switching state of the transmission ratio control valve 12. Specifically, the transmission ratio control valve 12 is switched into a direction in which the trunnion 7 is returned to the neutral position thereof by the actuator 10.
Therefore, the trunnion 7, just after it shifts in the axial direction, starts to shift in the opposite direction toward the neutral position. However, the trunnion 7 continues to swing about the pivot shafts 9, 9 as long as the shift thereof from the neutral position exists. As a result of this, the shift of the cam surface 21 of the precess cam 18 with respect to the circumferential direction is transmitted through the link arm 19 to the spool 15 to thereby shift the spool 15 in the axial direction. And, in a state where the inclination angle of the trunnion 7 reaches a given angle corresponding to a transmission ratio to be obtained, the trunnion 7 returns to the neutral position and, at the same time, the transmission ratio control valve 12 is closed, so that the supply of the pressure to the actuator 10 is stopped. As a result of this, the inclination angle of the trunnion 7 provides an angle corresponding to the amount by which the sleeve 14 has been shifted in the axial direction by the stepping motor 13.
The above-structured toroidal-type continuously variable transmission is in operation, one (in FIGS. 4 and 5, the left side) input side disk 2 is driven and rotated through such a pressing device 23 of a loading cam type or an oil pressure type as shown in FIGS. 4 and 5 by a drive shaft 22 which is connected to a drive source such as an engine. As a result of this, the pair of input side disks 2, 2 respectively supported on the two end portions of the input shaft 1 are rotated in synchronization with each other while they are pushed in their mutually approaching directions. And, these rotational movements are transmitted through the power rollers 6, 6 to the output side disks 5, 5 are then taken out from the output gear 4.
When transmitting the rotational movements from the input side disks 2, 2 to the output side disks 5, 5 in this manner, due to friction between the peripheral surfaces of the power rollers 6, 6 supported on the inner surfaces of the trunnions 7, 7 and the inner surfaces of the disks 2, 5, a force going in the axial direction of the pivot shafts 9, 9, which are disposed on the two end portions of the trunnions 7, 7, is applied to the trunnions 7, 7. This force is referred to as a so called 2Ft and the size of this force is proportional to the force (power) that is transmitted from the input side disks 2, 2 to the output side disks 5, 5 (or from the output side disks 5, 5 to the input side disks 2, 2). And, such force 2Ft is received by the actuators 10, 10. Therefore, when the toroidal-type continuously variable transmission is in operation, a pressure difference between a pair of oil pressure chambers respectively existing on the two sides of pistons 16, 16 constituting their associated actuators 10, 10 is proportional to the size of the force 2Ft.
Now, let us consider a case in which the rotation speed is changed between the input shaft 1 and output gear 4. Firstly, to reduce the speed between the input shaft 1 and output gear 4, the trunnions 7, 7 are respectively moved in the axial direction of the pivot shafts 9, 9 by the actuators 10, 10 and these trunnions 7, 7 are swung to such positions as shown in FIG. 5. And, the peripheral surfaces of the power rollers 6, 6 are respectively contacted with the near-to-center portions of the inner surfaces of the input side disks 2, 2 and the near-to-outer-periphery portions of the inner surfaces of the output side disks 5, 5 as shown in FIG. 5. On the other hand, to increase the rotation speed, the trunnions 7, 7 are swung in the opposite direction to FIG. 5 and the trunnions 7, 7 are respectively inclined in such a manner that the peripheral surfaces of the power rollers 6, 6, oppositely to the state shown in FIG. 5, are respectively contacted with the near-to-outer-periphery portions of the inner surfaces of the input side disks 2, 2 and the near-to-center portions of the inner surfaces of the output side disks 5, 5. In case where the inclination angles of these trunnions 7, 7 are set in the intermediate angles, there can be obtained an intermediate transmission ratio (a speed ratio) between the input shaft 1 and output gear 4.
Further, when a toroidal-type continuously variable transmission unit structured and operatable in the above-mentioned manner is actually assembled into a continuously variable transmission for a car, to construct a continuously variable transmission apparatus by combining the present toroidal-type continuously variable transmission unit with a planetary gear mechanism is conventionally proposed as disclosed in JP-A-1-169169, JP-A-1-312266, U.S. Pat. No. 5,888,160, U.S. Pat. No. 6,171,210 and the like.
Now, FIG. 7 shows a continuously variable transmission apparatus which is disclosed in U.S. Pat. No. 6,171,210 of the above-cited publications. This continuously variable transmission apparatus comprises a combination of a toroidal-type continuously variable transmission 24 of a double cavity type and a planetary-gear-type transmission 25. And, in the low speed running operation, the power is transmitted only by the toroidal-type continuously variable transmission 24; and, in the high speed running operation, the power is transmitted mainly by the planetary-gear-type transmission 25 and, at the same time, a speed ratio to be obtained by the planetary-gear-type transmission 25 can be adjusted freely by changing the speed ratio of the toroidal-type continuously variable transmission 24.
For the above purpose, the leading end portion (in FIG. 7, the right end portion) of an input shaft 1, which penetrates through the central portion of the toroidal-type continuously variable transmission 24 and also on the two end portions of which a pair of input side disks 2, 2 are supported, is connected through a high speed clutch 29 to a transmission shaft 28 which is fixed to the central portion of a support plate 27 supporting a ring gear 26 constituting the planetary-gear-type transmission 25. The structure of the toroidal-type continuously variable transmission 24 is substantially similar to the conventional structure previously shown in FIGS. 4 to 6, except for a pressing device 23a which will be described below.
Also, between the output side end portion (in FIG. 7, the right end portion) of a crankshaft 31 of an engine 30 serving as a drive source and the input side end portion (=base end portion=the left end portion in FIG. 7) of the input shaft 1, there are interposed a start clutch 32 and a pressing device 23a of an oil pressure type in such a manner that they are arranged in series to each other with respect to the transmission direction of the power. In the case of the continuously variable transmission apparatus disclosed in the above-cited U.S. Pat. No. 6,171,210, an arbitrary level of oil pressure can be introduced into the pressing device 23a. 
An output shaft 33 used to take out the power based on the rotation of the input shaft 1 is disposed concentrically with the input shaft 1. And, on the periphery of the output shaft 33, there is disposed the planetary-gear-type transmission 25. A sun gear 34, which constitutes the planetary-gear-type transmission 25, is fixed to the input side end portion (in FIG. 7, the left end portion) of the output shaft 33. Therefore, the output shaft 33 can be rotated as the sun gear 34 is rotated on the periphery of the sun gear 34, there is supported the ring gear 26 in such a manner that it is concentric with the sun gear 34 and can be rotated. And, between the inner peripheral surface of the ring gear 26 and the outer peripheral surface of the sun gear 34, there are interposed a plurality of planetary gears 35, 35. Each of the planetary gears 35, 35 is composed of a pair of planetary gear elements 36a, 36b. These planetary gear elements 36a, 36b are meshingly engaged with each other; and, the planetary gear element 36a disposed on the outside diameter side is meshingly engaged with the ring gear 26, whereas the planetary gear element 36b disposed on the inside diameter side is meshingly engaged with the sun gear 34. The thus-structured planetary gears 35, 35 are rotatably supported on the one side surface (in FIG. 7, the left side surface) of a carrier 37. Also, the carrier 37 is rotatably supported on the intermediate portion of the output shaft 33.
Also, the carrier 37 is connected through a power transmission mechanism 38 to a pair of output side disks 5, 5 constituting the toroidal-type continuously variable transmission 24 in such a manner that the transmission of the rotational force between them is possible. The power transmission mechanism 38 comprises a transmission shaft 39 arranged in parallel to the input shaft 1 and output shaft 33, a sprocket 40a fixed to one end portion (in FIG. 7, the left end portion) of the transmission shaft 39, a sprocket 40b fixed to the output side disks 5, 5, a chain 41 set over and between the two sprockets 40a, 40b, and first and second gears 42, 43 which are respectively fixed to the other end portion (in FIG. 7, the right end portion) of the transmission shaft 39 and to the carrier 37 and also are meshingly engaged with each other. Therefore, the carrier 37, as the output side disks 5, 5 are rotated, can be rotated in the opposite direction to these output side disks 5, 5 at the speed that corresponds not only to the number of teeth of the first and second gears 42, 43 but also to the pair of sprockets 40a, 40b. 
On the other hand, the input shaft 1 and ring gear 26 can be connected to each other through the transmission shaft 28 disposed concentrically with the input shaft 1 in such a manner that the transmission of the rotation power is possible between them. Between the transmission shaft 28 and input shaft 1, there is interposed the high speed clutch 29 in such a manner that it is arranged in series to the two shafts 28, 1. Therefore, when the high speed clutch 29 is connected, the transmission shaft 28 can be rotated in the same direction and at the same speed as the input shaft 1 as the input shaft 1 is rotated.
Also, the continuously variable transmission apparatus shown in FIG. 7 further includes a clutch mechanism which constitutes mode switching means. This clutch mechanism is composed of the high speed clutch 29, a low speed clutch 44 interposed between the outer peripheral edge portion of the carrier 37 and the axial-direction one end portion (in FIG. 7, the right end portion) of the ring gear 26, and a retreat clutch 45 interposed between the ring gear 26 and the fixed portion of the continuously variable transmission apparatus such as a housing (not shown). In the case of these clutches 29, 44, 45, when any one of them is connected, the connection of the remaining two clutches is cut off.
In the case of the above-structured continuously variable transmission apparatus, firstly, in the low speed running operation, not only the low speed clutch 44 is connected but also the connection of the high speed clutch 29 and retreat clutch 45 is cut off. In this state, when the start clutch 32 is connected and the input shaft 1 is rotated, only the toroidal-type continuously variable transmission 24 is allowed to transmit the power from the input shaft 1 to the output shaft 33. In such low speed running operation, the speed ratio between the pair of input side disks 2, 2 and the pair of output side disks 5, 5 may be adjusted similarly to the previously described structure which is shown in FIGS. 4 to 6 and also in which the toroidal-type continuously variable transmission is used singly.
On the other hand, in the high speed running operation, not only the high speed clutch 29 is connected but also the connection of the low speed clutch 44 and retreat clutch 45 is cut off. In this state, in case where the start clutch 32 is connected and the input shaft 1 is rotated, the power is transmitted from the input shaft 1 to the output shaft 33 by the transmission shaft 28 and planetary-gear-type transmission 25. That is, in case where the input shaft 1 is rotated in the high speed running operation, the rotational power of the input shaft 1 is transmitted through the high speed clutch 29 and transmission shaft 28 to the ring gear 26. And, the rotational power of the ring gear 26 is transmitted through the plurality of planetary gears 35, 35 to the sun gear 34, thereby rotating the output shaft 33 with the sun gear 34 fixed thereto. In this state, in case where the speed ratio of the toroidal-type continuously variable transmission 24 is changed to thereby change the revolving speed of the respective planetary gears 35, 35 around the sun gear 34, the speed ratio of the whole of the continuously variable transmission apparatus can be adjusted.
That is, in the above-mentioned high speed running operation, the respective planetary gears 35, 35 are revolved around the sun gear 34 in the same direction as the ring gear 26. And, the slower the revolving speeds of the planetary gears 35, 35 around the sun gear are, the faster the rotation speed of the output shaft 33 with the sun gear 34 fixed thereto is. For example, in case where the revolving speed of the planetary gears 35 around the sun gear 34 and the rotation speeds of the ring gear 26 (both of the speeds are angular speeds) become equal to each other, the rotation speeds of the ring gear 26 and output shaft 33 become equal to each other. On the other hand, in case where the rotation speeds of the planetary gears 35 are slower than the revolving speed of the ring gear 26, the rotation speed of the output shaft 33 is faster than the rotation speed of the ring gear 26. Contrary to this, in case where the rotation speed of the planetary gears 35 is faster than the revolving speed of the ring gear 26, the rotation speed of the output shaft 33 is slower than the rotation speed of the ring gear 26.
Therefore, in the high speed running operation, the more the speed ratio of the toroidal-type continuously variable transmission 24 is changed toward the speed reducing side, the more the speed ratio of the whole of the continuously variable transmission apparatus is changed to the speed increasing side. In such high speed running operation, power (torque) is applied to the toroidal-type continuously variable transmission 24 not from the input side disks 2, 2 but from the output side disk 5 (assuming that a torque to be applied in the low speed operation is a positive torque, a negative torque is applied). That is, in a state where the high speed clutch 29 is connected, the torque transmitted from the engine 30 to the input shaft 1 is transmitted through the transmission shaft 28 to the ring gear 26 of the planetary-gear-type transmission 25. Therefore, there exists little torque which is transmitted to the input side disks 2, 2 from the input shaft 1 side.
On the other hand, part of the torque transmitted through the transmission shaft 28 to the ring gear 26 of the planetary-gear-type transmission 25 is transmitted from the respective planetary gears 35, 35 through the carrier 37 and power transmission mechanism 38 to the respective output side disks 5, 5. The torque to be applied from the output side disks 5, 5 to the toroidal-type continuously variable transmission 24 decreases as the speed ratio of the toroidal-type continuously variable transmission 24 is changed toward the speed reducing side, in order that the speed ratio of the whole of the continuously variable transmission apparatus can be changed to the speed increasing side. As a result of this, the torque to be input to the toroidal-type continuously variable transmission 24 in the high speed running operation decreases. And, in case where the torque to be applied to the toroidal-type continuously variable transmission 24 is small in this manner, the pressing force to be generated by the pressing device 23a is reduced to thereby enhance the durability of the composing parts of the toroidal-type continuously variable transmission 24. (U.S. Pat. No. 6,171,210).
Further, when rotating the output shaft 33 reversely in order to back a car, not only the connection of both of the low speed and high speed clutches 44, 29 is cut off but also the retreat clutch 45 is connected. As a result of this, not only the ring gear 26 is fixed but also the respective planetary gears 35, 35 revolve around the periphery of the sun gear 34 while they are meshingly engaged with the ring gear 26 and sun gear 34. And, the sun gear 34 and output shaft 33 with the sun gear 34 fixed thereto are rotated in the opposite direction to the previously described low speed and high speed running operations.
As the structure of a pressing device used in a toroidal-type continuously variable transmission for securing the surface pressure of the rolling contact portions (traction portions) between the inner surfaces of the input side and output side disks and the peripheral surfaces of the respective power rollers, there are known, besides the structure shown in FIGS. 4, 5 and 7, structures which are disclosed in JP-B-6-72652 and JP-A-2000-65193. Of the two cited publications, JP-B-6-72652 discloses a structure which adjusts oil pressure to be introduced into a pressing device of an oil pressure type by manifold vacuum of the engine and inclined angle of the trunnion as well as a structure which combines together a loading cam and an oil pressure cylinder, allows the loading cam to generate a pressing force corresponding to an input torque, and allows the oil pressure cylinder to generate a pressing force corresponding to a transmission ratio. Also, in JP-A-2000-65193, there is disclosed a structure in which the kinematic viscosity of traction oil is measured using a viscosity sensor and a pressing force to be generated by the pressing device is changed according to the measured kinematic viscosity.
Of the above-mentioned conventional structures, in the case of the structure shown in FIGS. 4 and 5, the pressing force to be generated by the pressing device 23 of a loading cam type is often excessively large, which provides a disadvantage from the viewpoint of securing the durability of the composing parts of the toroidal-type continuously variable transmission 24. That is, for example, as disclosed in the above-cited publication, JP-B-6-72652 or “Motion & Control” NSK Technical Journal No. 10 Apr. 2001, it is conventionally known that the pressing force required of the pressing device 23 varies according to the transmission ratios. On the other hand, the pressing force to be generated by the pressing device 23 of a loading cam type is constant so long as the torque applied to the input portion of the pressing device 23 is the same. Therefore, the pressing device 23 of a loading cam type is designed in such a manner that it can generate the largest pressing force required. In concrete terms, in a case where the transmission ratio requires maximum pressing force, the pressing device is designed as a structure generating the required force. For this reason, in case where the transmission ratio deviates greatly from 1 (that is, in case where a speed increasing ratio or a speed reducing ratio becomes large), the pressing force to be generated by the pressing device 23 is excessively large. The excessive large pressing force is undesirable not only from the viewpoint of reduction in the size of the toroidal-type continuously variable transmission, for securement of transmission efficiency but also from the viewpoint of securing the durability of the composing parts of the toroidal-type continuously variable transmission.
Also, in the case of the structure shown in FIG. 7, consideration is given only to reduction in the oil pressure to be generated by the pressing device 23a when the torque passing through the toroidal-type continuously variable transmission 24 in the high speed mode in which the high speed clutch 29 is connected. Therefore, the present structure cannot always provide a sufficient effect from the viewpoint of securement of transmission efficiency and from the viewpoint of securement of durability.
Also, the structure disclosed in JP-B-6-72652 is a structure in which there is generated a pressing force with an input torque and a transmission ratio taken into consideration. However, it is difficult to make such a fine adjustment as to reduce sufficiently a difference between a pressing force required and a pressing force occurring actually.
Further, the structure disclosed in JP-A-2000-65193 is capable of obtaining a pressing force corresponding to the kinematic viscosity of the traction oil. However, the present structure is not able to make a more detailed adjustment. Also, not only it is difficult to measure the kinematic viscosity of the traction portion but also, even in case where such measurement is possible, it is unavoidable that the structure is complicated.
In order to reduce sufficiently the difference between the necessary pressing force and the actually occurring pressing force, that is, in order to make the pressing force to be generated by the pressing device correspond with the minimum pressing force necessary to secure the surface pressure of the traction portion (actually, in order to make the former slightly larger than the latter), it can be expected that the oil pressure to be introduced into the pressing device of an oil pressure type is controlled electrically. In case where the oil pressure is controlled electrically in this manner, regardless of variations in the transmission ratio, the pressing force to be generated by the pressing device can be made slightly larger than the minimum necessary pressing force, thereby being able not only to prevent the surface pressures of the traction portions from being excessively large but also to prevent excessive slippage from occurring in the traction portions.
However, in case where the oil pressure to be introduced into the pressing device of an oil pressure type is controlled purely electrically, when a computer for control is out of order or when a control circuit is out of order due to breaking of a wire, the oil pressure disappears or lowers extremely. As a result of this, the inner surfaces of the input side and output side disks and the peripheral surfaces of the power rollers, which are the composing parts of the toroidal-type continuously variable transmission, slip with respect to each other in their mutual rolling contact portions (that is, the above-mentioned traction portions), thereby causing so called gross slippage which makes it impossible to transmit the power between them. In case where such gross slippage occurs, not only a vehicle carrying the toroidal-type continuously variable transmission becomes unable to run but also the respective surfaces wear excessively, thereby raising a possibility that the toroidal-type continuously variable transmission can be damaged to such a degree that the transmission cannot be repaired. On the other hand, under present conditions, the possibility of an electrical control circuit being out of order is higher than the possibility of a control mechanism of an oil pressure type or a mechanical type being out of order. Therefore, in case where the oil pressure to be introduced into the pressing device of the above-mentioned oil pressure type is controlled only by the control circuit of a purely electrical type, there arises a problem from the viewpoint of securement of reliability.